This invention relates generally to reference voltage and reference current generator circuits and, more particularly, to reference current generator circuits having improved stability characteristics.
Reference voltage and reference current generator circuits produce constant and stable voltages and currents, respectively, for a variety of electronic circuit applications. A typical electronic circuit application employing a reference voltage generator circuit is current mode logic (CML). CML is a particular type of digital logic circuitry in which the transistors that form the logic gates are switched between the "on" and "off" states without becoming saturated. The transistors are prevented from saturating by limiting the base-to-collector forward junction voltages of the transistors. Because the transistors do not saturate, the switching speed of the transistors and, therefore, the speed of the CML circuit, is greatly increased.
Generally, two types of reference voltage generator circuits have been employed in the past with CML circuits. A reference voltage generator circuit is used in a CML circuit to drive the current source transistors, which supply the currents for the CML logic gate transistors. One type of reference voltage generator circuit used in the past is a simple arrangement of two bipolar transistors. A negative feedback loop extends from the collector of the second transistor, through the base and the emitter of the first transistor, to the base of the second transistor. Unfortunately this reference voltage generator circuit produces a reference voltage having a relatively large temperature coefficient. Furthermore, the currents generated by the current source transistors in a CML circuit are temperature dependent. Because the output voltages of the CML circuit are proportional to the currents supplied by the current source transistors, the output voltages of the CML circuit are also temperature dependent. This temperature dependency of the junction voltages of the CML logic gate transistors, which is aggravated at higher operating temperatures, can cause considerable saturation of the CML transistors. This decreases transistor switching time and, therefore, the speed of the CML circuit.
The second type of reference voltage generator circuit used in the past with CML circuits is a band-gap voltage reference generator circuit. The band-gap voltage reference generator circuit produces a reference voltage that is proportional to the bandgap voltage of silicon, which is approximately 1.23 Volts. The band-gap reference voltage generator circuit exploits both the negative and positive temperature coefficients of bipolar transistors. The temperature coefficient of the base-to-emitter junction voltage V.sub.BE of a bipolar transistor is negative, with a value of approximately -2 mV/.degree. C. The temperature coefficient of the voltage difference between the base-to-emitter junction voltages V.sub.BE of two bipolar transistors operating at different emitter current densities is positive. Combining the negative temperature coefficient of the base-to-emitter junction voltage with the positive temperature coefficient of the voltage difference between the base-to-emitter junction voltages produces a reference voltage having a nearly zero temperature coefficient. A typical band-gap voltage reference generator circuit is described in Hamilton, Douglas J. and Howard, William G., Basic Integrated Circuit Engineerinq, McGraw-Hill, Inc., 1975, at pages 429 to 431. However, ever, the currents generated by the current source transistors in a CML circuit are still temperature dependent and, therefore, the CML output voltages are also still temperature dependent. Accordingly, there has been a need for a reference voltage or reference current generator circuit that compensates for the temperature dependency of the currents generated by the current source transistors in a CML circuit. The present invention clearly fulfills this need.